1. Field of the Invention
This invention relates to an optical wavelength converting apparatus provided with an optical wavelength converting device for converting a fundamental wave into its second harmonic, or the like. This invention particularly relates to an optical wavelength converting apparatus, which is provided with an improved structure for mounting the optical wavelength converting device on a holder.
2. Description of the Prior Art
Various attempts have heretofore been made to convert the fundamental wave of a laser beam into its second harmonic, or the like, e.g. to shorten the wavelength of a laser beam, by using a crystal of a nonlinear optical material. As optical wavelength converting devices for carrying out such wavelength conversion, there have heretofore been known a bulk crystal type of optical wavelength converting device, an optical waveguide type of optical wavelength converting device, and the like.
By way of example, the optical wavelength converting device of this type is combined with a laser diode pumped solid laser or is employed in a structure comprising a semiconductor laser, which serves as a fundamental wave source, and an external resonator. In such cases, the optical wavelength converting device is ordinarily located in the region inside of the resonator. In general, the temperature of such a resonator is very accurately adjusted such that the resonator length may be kept constant. In such cases, the temperature of the optical wavelength converting device, which is located in the region inside of the resonator, is also adjusted.
The optical wavelength converting device described above is mounted on a holder and is secured at a predetermined position such that its optical axis may align with the optical axis of a laser, which serves as a fundamental wave source, a condensing lens, or the like. As the structure for mounting the optical wavelength converting device on the holder, two types of structures have heretofore been known. In one of the structures, the optical wavelength converting device is mounted on the holder by using a crystal pushing member, which is constituted of a metal, a plastic material, or the like. In the other structure, the optical wavelength converting device is adhered to the holder.
However, with the structure in which the crystal pushing member is utilized, the problems occur in that the optical wavelength converting device moves slightly due to a change in the environmental temperature, or the like, and cannot be secured at the correct position for a long period. In cases where the optical wavelength converting device is located in the region inside of the resonator, if the optical wavelength converting device thus moves to an incorrect position, the resonating conditions of the resonator will vary. As a result, the problems occur in that the intensity and the beam shape of the wavelength-converted wave fluctuate, and noise occurs.
With the structure in which the optical wavelength converting device is adhered to the holder, in cases where the holder is constituted of a metal, such as copper, the problems described below are encountered. Specifically, when a change in the environmental temperature occurs, a large stress is generated in the optical wavelength converting device due to a difference in the coefficient of thermal expansion between the optical wavelength converting device and the holder constituted of the metal. As a result, the optical wavelength converting device breaks or is distorted. A technique for adhering and securing a optical wavelength converting device to a metal material having a coefficient of thermal expansion close to the coefficient of thermal expansion of the optical wavelength converting device is disclosed in, for example, U.S. Pat. No. 5,150,376. However, even if the disclosed technique is applied to a structure for securing the optical wavelength converting device to the holder described above, satisfactory results cannot always be obtained.
Therefore, an attempt has been made to make the holder described above from glass, which has a coefficient of thermal expansion approximately equal to the coefficient of thermal expansion of the optical wavelength converting device. However, such a holder made from the material other than a metal has a low thermal conductivity. Therefore, in cases where the holder made from the material other than a metal is located in the region inside of the resonator, the accuracy, with which the temperature of the optical wavelength converting device is adjusted, cannot be kept high. As a result, the performance of the optical wavelength converting apparatus fluctuates.